[Footnote 1: Thorndike: _Educational Psychology_ (1910), p. 224.]
One other important correlation must be noted. While gifts and capacities are specific, superiority in a given trait commonly involves superiority in most others. Exceptional talent in one direction in most cases involves exceptionality in many other respects. While talents are not indiscriminately transferable from one field to another, the same complex of traits which makes a person stand out preëminently in a given field, say law, would make him stand out in any one of half a dozen different fields into which he might have gone. There seems to be no evidence that extraordinary capacity in one direction is balanced by extraordinary incapacity and stupidity in others. The fact that individuals differ not only in specific traits but in general mental capacity has, also, certain obvious practical consequences. It means that there are present in society, in the light of recent tests in the army, an unexpectedly large number of individuals below the level of normal intelligence. One in five hundred, Thorndike estimates, is the "frequency of intellectual ability so defective as to disturb the home, resist school influence, and excite popular derision." These are clearly liabilities in the social order. On the other hand, there is a large number above the level of average intelligence. The importance of this group for human progress can hardly be overestimated. As we have seen in other connections, progress is contingent upon variation from the "normal" or the accustomed, and such variation from the normal is initiated in the majority of cases by members of this comparatively small super-normal group. If civilization is to advance it must capitalize its intelligence; that is, educate up to the highest point of native ability. But in any case, its chief guarantee of progress lies in the comparatively small group in whom native ability is exceptionally high. For it is among this group that original thinking, invention, and discovery almost exclusively occur.
CAUSES OF INDIVIDUAL DIFFERENCES. Among the chief causes of individual differences may, in general, be set down the following: (1) Sex, (2) Race, (3) Near Ancestry or Family, (4) Environment. The particular fund of human nature which an individual displays, that is, his specific native endowments, as they appear in practice, will be a resultant of these various causes. In the study of each of these characteristics, we should be able ideally to eliminate all the others and to consider them each in isolation.